Is the gap you are filling too wide?
Is the bath slightly wobbly? Either may cause the cracking.
Consider which sealant you use - there are millions - but only a few for baths & showers. they will cost a bit more but worth it.
So if the gap is reasonable I put masking tape on the bath and other surface (tiles?), to leave the gap & abit more. Seal into this area and press & wipe with a damp finger to force into gap and smooth off. Press firmly on tape edge - or you will have a 'step' when you remove!
The smudges with be on the tape. Remove tape when satisfied but dont allow sealant to dry before doing so.
Note the instructions regarding CURING of sealant. Don't cut this time short before exposing to water!!! it is often 2 or more days.
It's a crap job - good luck!!
Steve
February 2007